In such systems, a gas generator rapidly generates the gas volume required for inflating the safety bag. The fuel is usually a solid fuel in the form of pellets which are ignited through an ignition device connected to a combustion chamber through ignition openings in the combustion chamber wall. The combustion chamber wall is also equipped with gas exit openings located opposite the ignition openings. The generated gas passes through the exit openings from the combustion chamber into a filter chamber which in turn is operatively connected to the interior of the safety bag for its inflation.
Conventional safety bag gas generators have a disadvantage in that injury may occur, especially to persons positioned close to the location of the gas generator which is located directly under or behind the dashboard. A child standing in front of the dashboard rather than sitting on the right-hand front seat, can thus be injured when it is in such an "out-of-position" when the gas generator is ignited. Numerous suggestions have been made to solve this problem, for example, by reducing the inflating speed by increasing the size of the gas generating fuel pellets. However, such an approach undesirably increases the total time needed for inflating the safety bag, thereby generating the danger that the safety bag will not be inflated sufficiently in time to perform its protection function. The safety bag must be fully inflated to be able to perform the protection function and such full inflation must be completed before the passenger or driver is caused to travel in the forward direction at high speed subsequent to an impact. The available time duration depends on the time it takes to crumple the so-called "crusher zone" of a vehicle following the beginning of an impact. Since such time duration is rather short, the reducing of the safety bag inflating speed is not a proper solution to the above mentioned problem.
Furthermore, a certain time duration of about 15 ms is needed following an impact to enable the sensors to evaluate whether the situation involved calls for the triggering of the inflation of the safety bag. The safety bag is not to be inflated in situations which do not constitute a critical impact. Thus, this time duration of about 15 ms further reduces the time available for inflating the safety bag. Thus, different requirements which oppose each other must be satisfied. On the one hand, during a first time phase following the beginning of an impact, a somewhat slower inflating of the safety bag is desirable for preventing injuries to a person close to the gas generator. On the other hand, a rapid inflating for the full deployment of the safety bag is desirable during a second time phase following an impact.
German Patent Publication (DE-OS) 2,224,201 discloses a gas generator with two combustion chambers filled with fuel for the gas generation. Each combustion chamber is provided with gas exit ports leading to the safety bag. As a result, a portion of the gas produced by the combustion of the fuel in the first combustion chamber is conducted directly into the safety bag, while the remaining portion of the generated inflating gas of the first combustion chamber ignites the fuel in the second combustion chamber in order to then completely inflate the safety bag. By inflating the safety bag in two stages it is intended to reduce the stress or load on the safety bag during its inflation and to also make sure that an excessive noise or noise pressure is not generated to avoid injuring a person's hearing. An inflating noise that would injure a person's hearing must be avoided. German Patent Publication (DE-AS) 2,915,202 discloses a gas generator of rotational symmetry suitable for improvement by the present invention.